The source being available is no reason not to write documentation. If everyone had to read the glibc and Linux kernel sources to write programs for Linux, we'd never have gotten anywhere.

Documentation is like sex — when it's good, it's very good, and when it's bad, it's still better than nothing.

The fact that POD makes it particularly easy to document means you really have no excuse not to write documentation. Imagine CPAN without POD, and half the modules without documentation: sure, there's all this great code out there, but if you want to know what it does you have to download and unpack it, and if there's no docs, read the source. How much use would such a repository be?

One thing that's cool about the hacker culture, however, is that even if you only wrote very sparse docs to begin with, if there's a lot of interest in your code it's likely that others will contribute documentation. They can also spot documentation errors for you, because they can read the source. Just remember that without any docs, it's not likely that anyone else will get interested. Make it habit to slap at least some basic POD onto your Perl code.